The present invention relates to a communication system adaptable to a radio communication, such as a mobile communication and a satellite communication. More particularly, this invention relates to a communication system, a transmitter, a receiver, and a communication method capable of reducing characteristics deterioration at compressed mode transmission in a CDMA (Code Division Multiple Access) communication system.
A conventional communication system will be explained here. For example, in the CDMA cellular system, the same carrier frequency is repetitively used within a cell, and handover between the frequencies is not necessary within the same cell. However, when the CDMA cellular system coresides with the existing system, handover between different carrier frequencies becomes necessary. Three concrete examples of handover between different carrier frequencies are given below.
The first example is the handover of the frequency between adjacent cells. When different carrier frequencies are assigned to adjacent cells, because the traffic is heavy due to of an increase of the number of subscribers, handover becomes necessary between such cells. The second example is the handover of the frequency between cells of the umbrella structure. For example, when the umbrella structure is formed, different carrier frequencies are assigned to large and small cells, and the handover is necessary between these cells. The third example is the handover of the frequency between the third generation system represented by the W(Wide)-CDMA system and the second generation system represented by the current cellular phone system.
When the handover takes place under the foregoing conditions, it is necessary to detect the power of different frequency carriers. In order to perform such detection, the receiver must be able to detect two frequencies. However, if the receiver is to detect two frequencies, the necessary structure makes the arrangement of the receiver either larger in size or complex in structure.
The handover method includes two types: handover led by a mobile station (Mobile Assisted Handover: MAHO) and handover led by a network (Network Assisted Handover: NAHO). In NAHO, the load on the mobile station is less as compared to MAHO, however, synchronization with each mobile station becomes necessary in the base station. Furthermore, in NAHO, in order to trace each mobile station separately, the arrangement of the base station/network becomes complex and huge.
Thus, MAHO is preferable from the point of view from the mobile station. However, in order to judge whether the handover should take place or not, intensities of two different frequency carriers have to be observed. Different from the TDMA (Time Division Multiple Access) system used in the second generation, the CDMA cellular system generally uses continuous transmission for both transmission and reception. Hence, in order to observe the intensities of two different frequency carriers, the transmission or reception timing has to be suspended to observe the other frequency unless a receiving device capable of handling two frequencies is prepared.
Accordingly, in the conventional communication system, a technique related to a compressed mode has been proposed, in which transmission information in the normal mode is time compressed, so that the compressed information is transmitted in a shorter time and the other frequency carrier is observed in the remaining time. One example is described in PCT Unexamined patent Publication No. 8-500475 entitled as xe2x80x9cDiscrete Transmission for Seamless Handover in DS-CDMA Systemxe2x80x9d. This publication discloses means to attain a compressed mode for cutting a data transmission time shorter by reducing a spreading factor of used spreading codes.
The compressed mode disclosed in the above publication will be explained in brief. FIG. 20 is a view showing a transmission example in the normal mode and compressed mode in the conventional CDMA cellular system. Vertical axis represents the power rate/transmission power and horizontal axis represents time. Compressed mode transmission is interposed between normal transmission frames. For example, in case of transmission in the compressed mode, a non-transmission time is set within a descending frame (compressed frame). The time length can be set arbitrarily. The non-transmission time is used as an idle time during which the intensity of the other frequency carrier is measured. As has been discussed, in the conventional CDMA cellular system, interposing the idle time between the compressed mode frame transmissions allows slot transmission.
Also, at the foregoing compressed mode transmission, the transmission power increases with a time ratio between the idle time and frame (compressed mode frame) transmission time. Thus, as is shown in FIG. 20, the compressed mode frame is transmitted at higher transmission power than the normal transmission frame. Consequently, it is possible to maintain the transmission quality at the frame transmission in the compressed mode.
Besides the foregoing publication, the references as to the compressed mode include Gustafsson, M. et. al., xe2x80x9cCompressed Mode Techniques for Inter-Frequency Measurements in a Wide-band DS-CDMA Systemxe2x80x9d, Proc. of 8th IEEE PIMRC, ""97. The latter publication discloses means to attain the compressed mode in case of increasing a coding rate, using multi-code transmission, or using multi-bit transmission modulation system, such as 16 QAM, besides the case of reducing the spreading factor.
On the other hand, in the conventional CDMA cellular system, in order to solve the xe2x80x9cperspective (near-and-far) problemxe2x80x9d that an undesired signal from a nearby station interferes with a desired signal from a remote station, transmission power control to the mobile station is effected so that the reception power in each base station will be equal. Hence, in the conventional CDMA cellular system, the channel state that is changed with time by adverse affect, such as fading, is corrected, so that not only can the required communication quality be secured at the receiver station, but also the line capacity can be utilized efficiently. The following description will describe the transmission power control in the conventional communication system with reference to the accompanying drawings.
FIG. 21 is a view showing the transmission power control at the normal mode transmission in the conventional communication system. To begin with, the receiver station determines reception power on the target such that meets the required communication quality, that is, target power. Here, the required communication quality on the target is not limited to the reception power, and may be a power ratio (SIR: Signal-to-Interference Ratio) between a desired signal and an interference signal instead. Then, the receiver station compares the power of the received desired signal with the target power, and if the former is greater than the latter, the receiver station sends a transmission power control command (TPC) to the transmitter station to lower the transmission power, and if the former is smaller than the latter, the receiver station sends a TPC command to the transmitter station to increase the transmission power. Upon receipt of the TPC command, the transmitter station changes the transmission power by using prescribed power amplitude: xcex94 in accordance with the content in the TPC command. Here, the transmission power control is performed per time unit called as a slot to follow a change in the channel state (channel state) shown in the drawing. Either a fixed value or a value that varies in accordance with a certain rule is given as the value of xcex94.
FIG. 22 is a view showing the transmission power control at the compressed mode transmission in the conventional communication system. The target power is not changed between the normal mode transmission and compressed mode transmission for ease of explanation. However, in general, there is a case that the set value of the target power is changed so as to ensure the required quality at the compressed mode transmission. Basic operations at the compressed mode transmission, such as following a change in the channel state, are the same as those at the normal mode transmission. However, at the compressed mode transmission, the receiver station does not receive a signal during the idle time in the compressed mode, and therefore, the receiver station cannot send a transmission power control command (TPC) to the transmitter station properly. This disables the transmission side to follow a change in the channel state, and when the transmission is resumed, a signal is sent on the transmission power immediately before shifting to the compressed mode as is shown in the drawing, thereby causing a xe2x80x9ctransmission power control errorxe2x80x9d. Accordingly, in the conventional communication system, the transmission power control error caused by the compressed mode transmission is converged as soon as possible by employing a method of increasing the power amplitude xcex94, for example. Hereinafter, a period since the transmission is resumed until the transmission power control error is converged (that is, a period until the reception power is restored in the vicinity of the target power) is referred to as a transmission power control convergence time.
Furthermore, in the conventional communication system, in order to achieve an interleave effect, the setting position of the idle time (non-transmission time) in the compressed mode is placed near the center of the frame formed by a plurality of slots as shown in FIG. 23, so that interleave is effected per base unit of a frame. In order to achieve a satisfactory interleave effect, it is more preferable to place the idle time near the center of the frame where bits within the frame can be dispersed with time than to place the idle time at the end of the frame to narrow the bits region after the interleave.
However, in the foregoing conventional communication system, a data volume within one frame is compensated with the actual transmitting time being compressed at the compressed mode transmission, and for this reason, a method of increasing a transmission rate by lowering the spreading factor or a method of increasing the transmission rate by reducing a coding rate is employed. Hence, as was discussed above as the prior art, in case that the idle time is placed near the center of the frame, slots with a lower spreading factor or slots with a reduced coding rate are placed in the transmission power control convergence time as shown in FIG. 23, which results in considerable deterioration in signal decoding accuracy. In short, the conventional communication system has a problem that adverse affect of the transmission power control error caused by the idle time is much greater than in the normal frame.
In addition, in the conventional communication system, in order to reduce the transmission power control error caused by the idle time, there has been proposed a method, in which the idle time is dispersed to more than one position so as to be placed discretely with time. However, according to the proposed method, each idle time is short, and when consideration is given to processing time or the like, there arises a problem that efficiency is reduced when observing the intensity of a different frequency carrier.
The present invention is devised to solve the above problems. It is an object of the present invention to obtain a communication system, a transmitter and a receiver, and a communication method capable of reducing adverse affect of the transmission power control error caused by the idle time without dispersing the idle time at the compressed mode transmission within a frame.
A communication system of the present invention, including a transmitter and a receiver both capable of operating in a normal mode or a compressed mode in which setting of a predetermined non-transmission time is allowed, the transmitter effecting transmission power control to a frame in each mode, wherein, when operating in the compressed mode, the transmitter changes a position of the non-transmission time in such a manner so as to minimize adverse affect of a transmission power control error that occurs after the non-transmission time.
According to the above-mentioned aspect, the position of the non-transmission time (idle time) in the compressed mode is changed in such a manner so as to minimize adverse affect of a transmission power control error that occurs after the non-transmission time, for example, with consideration given to the adverse affect of the transmission power control error and the interleave effect. Thus, different from the conventional method, it is not necessary to adapt a method of dispersing the non-transmission time at the compressed mode transmission within a frame.
According to the communication system of another aspect of the present invention, when the unit of interleaving is set to one frame, the transmitter places the non-transmission time in the compressed mode rearward from a center of a compressed frame.
According to the above-mentioned aspect, adverse affect of the transmission power control error is taken into consideration, and the position of the non-transmission time in the compressed mode is determined so as to be placed rearward from the center of the compressed frame, for example, thereby allowing observation of a different frequency carrier within the non-transmission time.
According to the communication system of another aspect of the present invention, the transmitter places data of at least one slot after the non-transmission time within the compressed frame, so that a satisfactory interleave effect is achieved.
According to the above-mentioned aspect, the position of the non-transmission time in the compressed mode is determined so as to be placed rearward from the center of the compressed frame, for example, with consideration given to the adverse affect of the transmission power control error and the interleave effect, and further, data of at least one slot is placed after the non-transmission time within the compressed frame, thereby allowing observation of a different frequency carrier within the non-transmission time.
According to the communication system of another aspect of the present invention, when the unit of interleaving is set to one frame and the non-transmission time extends over two frames, the transmitter places the non-transmission time in the compressed mode longer in a first frame and set relatively shorter in a second frame.
According to the above-mentioned aspect, the non-transmission time is set relatively longer in a first frame and set relatively shorter in a second frame, so that a satisfactory interleave effect can be achieved even when the non-transmission time extends over first and following second frames with consideration given to adverse affect of the transmission power control error to the second frame.
According to the communication system of another aspect of the present invention, the receiver estimates maximum Doppler frequency, compares the estimated maximum Doppler frequency with a preset threshold of the maximum Doppler frequency, and when the estimated maximum Doppler frequency is higher than the threshold, negotiates with the transmitter not to effect control as to a change of the position of the non-transmission time and when the estimated maximum Doppler frequency is lower than the threshold, the transmitter places the non-transmission time in the compressed mode rearward from the center of the compressed frame.
According to the above-mentioned aspect, the estimated value of the maximum Doppler frequency is compared with the preset threshold of the maximum Doppler frequency, and when a frequency of the estimated value is lower than the threshold, the non-transmission time is placed rearward in the compressed frame. On the other hand, when the frequency of the estimated value is higher than the threshold, negotiation is made so as not to adjust the non-transmission time, and the non-transmission time is placed near the center of the compressed frame. In other words, the position of the non-transmission time in the compressed frame is changed in response to the pitch of the fading frequency.
According to the communication system of another aspect of the present invention, the transmitter and receiver set a step size of power in transmission power control larger than a predetermined value set as a reference value through negotiation, and reduce a number of slots needed for the transmission power control error convergence that occurs after the non-transmission time.
According to the above-mentioned aspect, the step size of the transmission power control is determined in response to the fading frequency, and further, by estimating the transmission power control error convergence time based on the step size, the non-transmission time is set with consideration given to the adverse affect of the transmission power control error caused by the non-transmission time and the interleave effect.
According to the communication system of another aspect of the present invention, in an area where moving at a high speed is expected, the control as to a change of the position of the non-transmission time is not effected, and in an area where moving at a high speed is not expected, the non-transmission time in the compressed mode is placed rearward from the center of the compressed frame.
According to the above-mentioned aspect, by estimating the fading frequency based on the largeness of the cell radius, the non-transmission time is set with consideration given to the adverse affect of the transmission power control error caused by the non-transmission time and the interleave effect.
A transmitter of another aspect of the present invention for operating in a normal mode or a compressed mode in which setting of a predetermined non-transmission time is allowed and effecting transmission power control to a frame in each mode wherein, when operating in the compressed mode, changing a position of the non-transmission time in such a manner so as to minimize adverse affect of a transmission power control error that occurs after the non-transmission time.
According to the above-mentioned aspect, the position of the non-transmission time in the compressed mode is changed in such a manner so as to minimize the adverse affect of the transmission power control error that occurs after the non-transmission time, for example, with consideration given to the adverse affect of the transmission power control error and the interleave effect.
According to the transmitter of another aspect of the present invention, when the unit of interleaving is set to one frame, placing the non-transmission time in the compressed mode rearward from a center of a compressed frame.
According to the above-mentioned aspect, adverse affect of the transmission power control error is taken into consideration, and the position of the non-transmission time in the compressed mode is determined so as to be placed rearward from the center of the compressed frame, for example, thereby allowing observation of a different frequency carrier.
According to the transmitter of another aspect of the present invention, placing data of at least one slot after the non-transmission time within the compressed frame, so that a satisfactory interleave effect is achieved.
According to the above-mentioned aspect, the position of the non-transmission time in the compressed mode is determined so as to be placed rearward from the center of the compressed frame, for example, with consideration given to the adverse affect of the transmission power control error and the interleave effect, and further, data of at least one slot is placed after the non-transmission time within the compressed frame, thereby allowing observation of a different frequency carrier.
According to the transmitter of another aspect of the present invention, when the unit of interleaving is set to one frame and the non-transmission time extends over two frames, placing the non-transmission time in the compressed mode longer in a first frame and set relatively shorter in a second frame.
According to the above-mentioned aspect, the non-transmission time is set relatively longer in a first frame and set relatively shorter in a second frame, so that a satisfactory interleave effect can be achieved even when the non-transmission time extends over first and following second frames with consideration given to the adverse affect of the transmission power control error to the second frame.
According to the transmitter of another aspect of the present invention, through negotiation with a receiver, setting a step size of power in transmission power control larger than a predetermined value set as a reference value, and reducing a number of slots needed for the transmission power control error convergence that occurs after the non-transmission time.
According to the above-mentioned aspect, the step size of the transmission power control is determined in response to the fading frequency, and further, by estimating the transmission power control error convergence time based on the step size, the non-transmission time is set with consideration given to the adverse affect of the transmission power control error caused by the non-transmission time and the interleave effect.
A receiver of another aspect of the present invention estimates maximum Doppler frequency, compares the estimated maximum Doppler frequency with a preset threshold of the maximum Doppler frequency, and when the estimated maximum Doppler frequency is higher than the threshold, negotiates with a transmitter not to effect control as to a change of the position of the non-transmission time.
According to the above-mentioned aspect, the estimated value of the maximum Doppler frequency is compared with the preset threshold of the maximum Doppler frequency, and when a frequency of the estimated value is higher than the threshold, negotiation is made so as not to adjust the non-transmission time, and the non-transmission time is placed near the center of the compressed frame.
According to the receiver of another aspect of the present invention, through negotiation with a transmitter, setting a step size of power in transmission power control larger than a predetermined value set as a reference value, and reducing a number of slots necessary for a transmission power control error that occurs after the non-transmission time to converge.
According to the above-mentioned aspect, the step size of the transmission power control is determined in response to the fading frequency, and further, by estimating the transmission power control error convergence time based on the step size, the non-transmission time is set with consideration given to the adverse affect of the transmission power control error caused by the non-transmission time and the interleave effect.
A communication method of another aspect of the present invention comprises a transmission step and a reception step operating in a normal mode or a compressed mode in which setting of a predetermined non-transmission time is allowed, the transmission step including effecting of transmission power control, wherein, when operating in the compressed mode, a position of the non-transmission time is changed in the transmission step in such a manner so as to minimize adverse affect of a transmission power control error that occurs after the non-transmission time.
According to the above-mentioned aspect, the position of the non-transmission time in the compressed mode is changed in such a manner so as to minimize adverse affect of a transmission power control error that occurs after the non-transmission time, for example, with consideration given to the adverse affect of the transmission power control error and the interleave effect.
According to the communication method of another aspect of the present invention, when the unit of interleaving is set to one frame, in the transmission step, the non-transmission time in the compressed mode is placed rearward from a center of a compressed frame.
According to the above-mentioned aspect, adverse affect of the transmission power control error is taken into consideration, and the position of the non-transmission time in the compressed mode is determined so as to be placed rearward from the center of the compressed frame, for example, thereby allowing observation of a different frequency carrier.
According to the communication method of another aspect of the present invention, in the transmission step, data of at least one slot is placed after the non-transmission time within the compressed frame, so that a satisfactory interleave effect is achieved.
According to the above-mentioned aspect, the position of the non-transmission time in the compressed mode is determined so as to be placed rearward from the center of the compressed frame, for example, with consideration given to the adverse affect of the transmission power control error and the interleave effect, and further, data of at least one slot is placed after the non-transmission time within the compressed frame, thereby allowing observation of a different frequency carrier.
According to the communication method of another aspect of the present invention, when the unit of interleaving is set to one frame and the non-transmission time extends over two frames, in the transmission step, the non-transmission time in the compressed mode is set relatively longer in a first frame and set relatively shorter in a second frame.
According to the above-mentioned aspect, the non-transmission time is set relatively longer in a first frame and set relatively shorter in a second frame, so that a satisfactory interleave effect can be achieved even when the non-transmission time extends over first and following second frames with consideration given to adverse affect of the transmission power control error to the second frame.
According to the communication method of another aspect of the present invention, in the reception step, a maximum Doppler frequency is estimated, and the estimated maximum Doppler frequency is compared with a preset threshold of the maximum Doppler frequency, and when the estimated maximum Doppler frequency is higher than the threshold, a negotiation is made with a transmitter not to effect control as to a change of the position of the non-transmission time; and in the transmission step, when the frequency of the estimated maximum Doppler frequency is lower than the threshold, the non-transmission time in the compressed mode is placed rearward from the center of the compressed frame.
According to the above-mentioned aspect, the estimated value of the maximum Doppler frequency is compared with the preset threshold of the maximum Doppler frequency, and when a frequency of the estimated value is lower than the threshold, the non-transmission time is placed rearward in the compressed frame. On the other hand, when the frequency of the estimated value is higher than the threshold, negotiation is made so as not to adjust the non-transmission time, and the non-transmission time is placed near the center of the compressed frame.
According to the communication method of another aspect of the present invention, in the transmission step and reception step, a step size of power in transmission power control is set larger than a predetermined value set as a reference value through negotiation, and a number of slots needed for the transmission power control error convergence that occurs after the non-transmission time is reduced.
According to the above-mentioned aspect, the step size of the transmission power control is determined in response to the fading frequency, and further, by estimating the transmission power control error convergence time based on the step size, the non-transmission time is set with consideration given to the adverse affect of the transmission power control error caused by the non-transmission time and the interleave effect.
According to the communication method of another aspect of the present invention, in an area where moving at a high speed is expected, the control as to a change of the position of the non-transmission time is not effected, and in an area where moving at a high speed is not expected, the non-transmission time in the compressed mode is placed rearward from the center of the compressed frame.
According to the above-mentioned aspect, by estimating the fading frequency based on the largeness of the cell radius, the non-transmission time is set with consideration given to the adverse affect of the transmission power control error caused by the non-transmission time and the interleave effect.